On the Holocene Evolution of the Ayeyawady Megadelta 2 3 Authors: 4 5 Liviu Giosan1, Thet Naing2, Myo Min Tun3, Peter D

1 On the Holocene Evolution of the Ayeyawady Megadelta 2 3 Authors: 4 5 Liviu Giosan1, Thet Naing2, Myo Min Tun3, Peter D. Clift4, Florin Filip5, 6 Stefan Constantinescu6, Nitesh Khonde1,7, Jerzy Blusztajn1, Jan-Pieter Buylaert8, 7 Thomas Stevens9, Swe Thwin10 8 9 Affiliations: 10 11 1Geology & Geophysics, Woods Hole Oceanographic, Woods Hole, USA 12 2Pathein University, Pathein, Myanmar 13 3University of Mandalay, Mandalay, Myanmar 14 4Geology & Geophysics, Louisiana State University, USA 15 5The Institute for Fluvial and Marine Systems, Bucharest, Romania 16 6Geography Department, Bucharest University, Bucharest, Romania 17 7Birbal Sahni Institute of Palaeosciences, Lucknow, India 18 8Technical University of Denmark, Roskilde, Denmark 19 9Uppsala University, Uppsala, Sweden 20 10Mawlamyine University, Mawlamyine, Myanmar 21 22 23 24 25 26 Correspondence to: L. Giosan ([email protected]) 27 28 29 30 31 32 33 34 35 36 *submitted to Earth Surface Dynamics 37

1 38 Abstract: 39 40 The Ayeyawady delta is the last Asian megadelta whose evolution has remained 41 essentially unexplored so far. Unlike most other deltas across the world, the Ayeyawady 42 has not yet been affected by dam construction providing a unique view on largely natural 43 deltaic processes benefiting from abundant sediment loads affected by tectonics and 44 monsoon hydroclimate. To alleviate the information gap and provide a baseline for future 45 work, here we provide a first model for the Holocene development of this megadelta 46 based on radiocarbon and optically stimulated luminescence-dated trench and drill core 47 sediments collected in 2016 and 2017, together with a re-evaluation of published maps, 48 charts and scientific literature. Altogether, this data indicates that Ayeyawady is a mud- 49 dominated delta with tidal and wave influences. The sediment-rich Ayeyawady River 50 built meander belt alluvial ridges with avulsive characters. A more advanced coast in the 51 western half of delta (i.e., the Pathein lobe) was probably favored by the more western 52 location of the early course of the river. Radiogenic isotopic fingerprinting of the 53 sediment suggest that the Pathein lobe coast does not receive significant sediment from 54 neighboring rivers. However, the eastern region of the delta (i.e., Yangon lobe) is offset 55 inland and extends east into the mudflats of the Sittaung estuary. Wave-built beach ridge 56 construction during the late Holocene, similar to other several deltas across the Indian 57 monsoon domain, suggests a common climatic control on monsoonal delta 58 morphodynamics through variability in discharge, changes in wave climate, or both. 59 Correlation of the delta morphological and stratigraphic architecture information onland 60 with the shelf bathymetry, as well as its tectonic, sedimentary and hydrodynamic 61 characteristics provide insight on the peculiar growth style of the Ayeyawady delta. The 62 offset between the western Pathein lobe and the eastern deltaic coast appears to be driven 63 by tectonic-hydrodynamic feedbacks as the extensionally lowered shelf block of the Gulf 64 of Mottama amplifies tidal currents relative to the western part of the shelf. This situation 65 probably activates a perennial shear front between the two regions that acts as a leaky 66 energy fence. Just as importantly, the strong currents in the Gulf of Mottama act as an 67 offshore-directed tidal pump that help build the deep mid-shelf Mottama clinoform with 68 mixed sediments from Ayeyawady, Sittaung, and Thanlwin rivers. The highly energetic 69 tidal, wind and wave regime of the northern Andaman Sea thus exports most sediment 70 offshore despite the large load of the Ayeyawady river. 71

2 72 Introduction 73 74 Asian megadeltas (Woodroffe et al., 2006) have a long history of human habitation and 75 anthropogenic impact. With large populations, which increasingly congregate in 76 sprawling megacities, these vast low-lying and ecologically-rich regions are under threat 77 from environmental degradation, climate change and sea level rise. The Ayeyawady 78 (formerly known as Irrawaddy or Ayeyarwady) is the least studied of these megadeltas 79 despite its scientific, social and economic importance (Hedley et al., 2010). Located in 80 the larger India-Asia collision zone, the Ayeyawady delta (Fig. 1) bears the imprint of 81 uniquely complex tectonic processes in a region of oblique subduction (Morley et al., 82 2017) and is a repository for unusually large sediment yields under an erosion-prone 83 monsoon climate (e.g., Giosan et al., 2017). Sediment redistribution within the delta and 84 on the shelf fronting it is affected by strong tides amplified by the geomorphology of the 85 region (Ramasawamy and Rao, 2014). In contrast to other Asian megadeltas, the 86 Ayeyawady river basin is arguably less transformed by post-World War II anthropogenic 87 impacts, although humans have probably affected delta development since at least the 88 Iron Age as agriculture expanded along the river (Moore, 2007) and later intensified 89 during the Pyu (~200 BC to 1050 AD), Bagan (~850 to 1300 AD), and Ava (~1350 to 90 1550 AD) periods. Recent rapid development trends and population growth underline the 91 need to understand the history and document the current state of the Ayeyawady delta. 92 93 Although the Ayeyawady River is less regulated compared to other large rivers, plans are 94 afoot to construct several dams across it and this may change the water and sediment 95 regimes, as well as fluxes reaching its low-lying delta plain (Brakenridge et al., 2017). 96 Inundation of the Ayeyawady delta region during cyclone Nargis in 2008 was one of the 97 costliest and deadliest natural disasters ever recorded (Fritz et al., 2009; Seekins, 2009). 98 Catastrophic monsoon-driven river floods are also common and devastating (Brakenridge 99 et al., 2017). The Ayeyawady delta may already be sediment deficient (Hedley et al., 100 2010) and the anticipated sediment deficit after damming could increase its vulnerability 101 to such transient events as well as to long term sea level rise (Giosan et al., 2014). Strong 102 tidal currents in the northern Andaman Sea (Rizal et al., 2012) amplify some aspects of 103 delta vulnerability, such as salinization (Taft and Evers, 2016) whereas other aspects may 104 be attenuated such as sediment redistribution along the coast or sediment trapping within 105 the subaerial delta (e.g., Hoitink et al., 2017). Better knowledge on how the delta has 106 formed and functioned will help future efforts to maintain its viability. 107 108 To alleviate the information gap and provide a baseline for future work we sketch here a 109 first model for the Holocene evolution of the Ayeyawady delta based on new field data 110 collected in two expeditions in 2016 and 2017 (Figs. 2 and 3; see Fig. S1 for site 111 locations and names) together with a re-evaluation of published maps, charts and 112 scientific literature (Figs. 4 and 5). In the process we reassess our knowledge concerning

3 113 monsoonal deltas in general by advancing new ideas on how morphodynamics and 114 sedimentary architecture can be controlled by feedbacks between tectonics and tides, as 115 well as by the balance between fluvial discharge and wave climate. 116 117 Background 118 119 The Ayeyawady River is a major fluvial system that became individualized in 120 Oligocene/Early Miocene time (Fig. 1; Licht et al., 2016; Morley, 2017 and references 121 therein). The Late Cretaceous subduction of the Neotethys Ocean followed by the 122 collision between India and Asia first led to an Andean-type margin comprised of the 123 Wuntho-Popa Volcanic Arc and associated forearc and backarc basins (e.g., Racey and 124 Ridd, 2015; Liu et al., 2016). The uplift of the Indo-Burman Ranges accretionary prism 125 since early Paleogene completed the separation of the Central Myanmar Basin (CMB) 126 from the Bay of Bengal. The complex of basins forming the CMB were further 127 segmented by compression and inversion (e.g., Bender, 1983). These basins include the 128 Ayeyawady Valley separated by the Bago Yoma (Pegu Yoma) from the Sittaung 129 (Sittang) Valley flowing along the Shan Plateau. The Ayeyawady River infilled this ~900 130 km long shallow marine area toward the Andaman Sea, a Cenozoic backarc/strike-slip 131 basin induced by oblique subduction of the Indian plate under Eurasia (e.g., Curray, 132 2005). A southern shift in Ayeyawady deposition was evident in the Miocene after the 133 major strike-slip fault, the Sagaing, activated along Bago Yoma. The Holocene delta is 134 the last realization in a series of deltas comprising this southward-moving Ayeyawady 135 depocenter. 136 137 Myanmar’s hydroclimate that is responsible for Ayeyawady flow is spatially complex 138 owing to its varied topography and compound influences from both the Indian and East 139 Asian monsoon systems (Brakenridge et al., 2017). Orographic precipitation occurs 140 along the northeastern Himalayas and Indo-Burman Ranges (Xie et al., 2006), as well as 141 the Shan Plateau feeding the upper Ayeyawady and the Chindwin, whereas Central 142 Myanmar, in the lee of these ranges, remains drier. The upper basin of the Ayeyawady 143 also receives snow and glacier meltwater in the spring. Over 90% of the discharge at the 144 delta occurs between May and October with small but significant interannual variability 145 (Furuichi et al., 2009) linked to the El Nino-Southern Oscillation, Indian Ocean Dipole, 146 and Pacific Decadal Oscillation (D’Arigo and Ummenhofer, 2014 and references therein). 147 148 In historical times the Ayeyawady River has transported ~422 ± 41 × 109 m3 of 149 freshwater every year to the ocean (Robinson et al, 2007), watering Myanmar from north 150 to south along the way (Fig. 1). The water discharge has now apparently decreased to the 151 present level of 379 ± 47 × 109 m3/year (Furuichi et al., 2009). Among the delta-building 152 Himalayan rivers, the Ayeyawady is a prodigious sediment conveyor (~364 ± 60 × 106

4 153 t/year), second only to the combined Ganges-Brahmaputra (Robinson et al, 2007). 154 Between 40 and 50% of the sediment comes from the upper Ayeyawady, with the rest 155 supplied by its main tributary, the Chindwin (Garzanti et al., 2016). Sediments 156 transported by the Upper Ayeyawady River come primarily from erosion of gneisses and 157 granitoids of the Himalayan Eastern Syntaxis region and the Sino-Burman Ranges. 158 Although draining less steep terrain, the Chindwin contributes more sediment than the 159 Upper Ayeyawady from the easily erodible flysch and low-grade metasedimentary rocks 160 of the Indo-Burman Ranges. Both water and sediment discharge vary synchronously at 161 interannual time scales as a function of monsoon intensity (Furuichi et al., 2009), but 162 they changed little since the late 19th century when Gordon (1893) measured them 163 systematically for the first time. In addition to the Ayeyawady, the Sittaung River 164 supplies sediment to the northern shore of the Gulf of Mottama (aka Gulf of Martaban) 165 where its estuary merges with the Ayeyawady delta coast (Fig. 1). The sediment 166 discharge from the Sittaung is unknown but can be estimated based on its annual water 167 discharge range of 50 × 109 m3/year to a maximum of 40 to 50 × 106 t/year by assuming 168 sediment yields similar to the Ayeyawady (Milliman and Farnsworth, 2010). Another 169 sediment contributor to the Gulf of Mottama (∼180 × 106 t/year) is the Thanlwin River 170 (Salween) draining the eastern Shan Plateau and eastern Tibetan Plateau (Robinson et al, 171 2007). Information about the variability in Ayeyawady’s sediment discharge over the 172 Holocene lifetime of the delta is sparse, as are reconstructions for the monsoon variability 173 in its basin. Assuming the modern direct correlation between water discharge and 174 sediment load one may qualitatively infer an increase in sediment delivery since 10,000 175 years ago with a peak in around 5000 years ago when the Andaman Sea was at its 176 freshest (Gebregeorgis et al., 2016), followed by a decrease to the present values, as the 177 Indian monsoon has weakened since the mid Holocene (e.g., Ponton et al., 2012; Dixit et 178 al., 2014). 179 180 The Ayeyawady delta is a mud-dominated delta that exhibits mainly tidal and secondarily 181 wave influences (Figs. 2 and 5; Kravtsova et al., 2009). Ayeyawady’s single braided 182 channel starts to show avulsive behaviour near the town of Myan Aung (~18.2°N) where 183 the tidal influence is still felt ~290 km from the Andaman Sea (Fig. 1). The apex of the 184 delta, defined as the region of deltaic distributary bifurcation, is north of the town of 185 Hinthada (18°N) around 270 km from the coast. Multiple branches are active in the delta, 186 splitting and rejoining to form a network of lower order distributary channels and 187 reaching the coast through eleven tidally-enlarged estuaries (Fig. 2). Most of the water 188 discharge (76%) is delivered to the Andaman Sea through three main mouths: Pyamalaw, 189 Ayeyawady and To-Thakutpin from west to east (Kravtsova et al., 2009). 190 191 In natural conditions when the delta was covered by tropical forests and mangroves 192 (Adas, 2011), sedimentation on the delta plain occurred within active and abandoned

5 193 channels, on channel levees and inter-distributary basins (Stamp, 1940; Kravtsova et al., 194 2009). The coast prograded via shoal/bar emergence and wave-built beach ridges with 195 associated interridge swales (Kravtsova et al., 2009). The coastline for the Ayeyawady 196 delta proper stretches from the western rocky Cape Maw Deng, adjacent to the Pathein 197 River, to the Yangon River in the east (Fig. 1). However, this conventional definition 198 does not capture the fact that the accumulative coast with sediment input from the 199 Ayeyawady continues east of the Yangon River into the Sittaung estuary. Despite the 200 large annual fluvial sediment load of the combined Ayeyawady and Sittaung (350–480 × 201 106 t), shoreline changes have been puzzlingly minor along the Ayeyawady delta coast 202 since 1850 (Hedley et al., 2010). Sea level change data is sparse and unreliable for the 203 delta and no data on subsidence/uplift exists. 204 205 The shelf morphology in front of the Ayeyawady delta is complex due to its tectonic 206 structure and the nature of Holocene sedimentation (Rodolfo, 1969a,b, 1975; 207 Ramaswamy and Rao, 2014). The width of the shelf is ~170 km wide off the Ayeyawady 208 River mouths, widening to more than 250 km in the Gulf of Mottama (Figs. 1 and 5). The 209 shelf edge exhibits a flat, platform-like indentation in the Gulf of Mottama between 140 210 and 180 m deep (i.e., the Martaban Depression - Ramaswamy and Rao, 2014) that 211 features a dendritic network of channels feeding the Martaban Canyon (Rodolfo, 1975). 212 Most of the large Ayeyawady suspended sediment load is redistributed by the strong tidal 213 currents (Fig. 5) and seasonally-reversing wind-currents to be deposited on the wide 214 northern Andaman shelf (Ramaswamy and Rao, 2014) where it mixes with sediment 215 from the Sittaung, Thanlwin and other smaller rivers (Damodararao et al., 2016). Semi- 216 diurnal tides vary between 2 and 3 m from the Pathein River to the Bogale River, 217 reaching higher stages inside distributaries. The tidal range is gradually amplified to 218 macrotidal conditions on the shallow (<30 m) shelf of the Gulf of Mottama from the 219 Bogale Promontory toward the Sittaung estuary where it reaches above 7 m during spring 220 tides (British Admiralty, 1935). Associated tidal currents also vary accordingly to over 221 3.5 m/s near the Sittaung mouth. 222 223 Waves are subordinate in importance for sediment transport to tides, with average heights 224 less than 1 m in winter to 1–2 m in summer (Kravtsova et al., 2009). Tidal currents 225 combine with the wind-driven circulation that is clockwise during the summer monsoon 226 and reversed during the winter monsoon (Rizal et al., 2012). The macrotidal regime 227 maintains turbid conditions year-round with the turbidity front oscillating ~150 km 228 offshore in the Gulf of Mottama in phase with the spring-neap tidal cycle (Ramaswamy et 229 al. 2004). Annual turbidity levels and suspended sediment distribution are modulated by 230 the monsoonal-driven winds, currents and river discharge (Ramaswamy et al. 2004; 231 Matamin et al, 2015) with the most extensive and compact turbid waters occurring in the 232 boreal winter. During the summer the turbidity region shrinks to the Gulf of Mottama and

6 233 nearshore regions where river plumes are active and dispersed eastward. Turbidity 234 profiles show an increase with depth during fair-weather and uniform concentrations 235 during major storms or cyclones (Ramaswamy et al. 2004; Shi and Wang 2008). Bottom 236 nepheloid layers and possibly hyperpycnal flows occur in the Gulf of Mottama and flow 237 into the interior of the Andaman Sea as mid-water nepheloid layers (Ramaswamy et al., 238 2004). 239 240 The bathymetric characteristics of the shelf and the circulation system favor deposition of 241 finer fluvial sediments in a mudbelt that widens from the western edge of the Ayeyawady 242 coast into the Gulf of Mottama that more or less coincides in extent with the high 243 turbidity region (Ramaswamy and Rao, 2014). The outer shelf, including the Martaban 244 Depression, is a zone of low to non-deposition, and exhibits a relict morphology with 245 topographic irregularities that host relict coarse-grained carbonate-rich sediment and 246 fauna with patchy Holocene muds (Ramaswamy and Rao, 2014). 247 248 In terms of human impacts on the delta it is important to note that the population of 249 Myanmar increased from 4–5 million in the late 19th century to ~51 million in 2014 with 250 30% residing in the Ayeyawady delta region. This large increase in population led to a 251 rapid rate of deforestation in the basin, but also to destruction of mangroves for 252 agriculture and fuel in the delta (Taft and Evers, 2016). An earlier large migration wave 253 to the delta occurred in the latter half of the 19th century when the British colonial 254 authorities cleared much of the delta forests and mangroves for rice agriculture (Adas, 255 2011). Construction of dikes to protect agricultural lands in the delta began in 1861 and 256 continued aggressively until the 1920s. These dikes are generally of a horseshoe type 257 protecting delta islands in the upstream and sides from floodwaves but recently poldering 258 with diking entire islands was employed. Most channels remain natural with no extensive 259 system of dredged canals. However, all dikes limit overbank flooding and deposition of 260 sediment (Volker 1966; Stamp 1940) and the entire agricultural system favors salinization 261 of soils in the delta. The model for the Holocene evolution of the Ayeyawady delta that 262 we provide below allows us to assess first order relationships to the complex regional 263 tectonics, climate, and shelf circulation as a baseline for the future development and 264 management of the delta. 265 266 Methods 267 268 The large scale morphology of the Ayeyawady delta, together with the adjoining regions 269 (Fig. 2), were assessed and studied using satellite data and old maps of the region. High- 270 resolution (90-m) digital elevation data were derived from NASA’s Shuttle Radar 271 Topography Mission (SRTM; Farr et al., 2007). Digital elevation models (DEMs) were 272 constructed at 300 m resolution and were used in combination with Advanced 273 Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Google Earth to

7 274 identify geomorphic features that provide insight into fluvial morphodynamics. The delta 275 and upstream floodplain was delimited from adjacent hinterlands with associated 276 marginal alluvial fans, as were remnant inselberg-like pre-deltaic terrains inside the delta. 277 We identified active and abandoned river courses and delta distributaries and their 278 meander belts. Finally, we identified fossil beach ridges denoting former delta shorelines. 279 Guided by this assessment, in two field expeditions in the Ayeyawady delta in 2016 and 280 2017, we collected sedimentary records from shallow hand-dug trenches and cores with 281 mechanized pneumatic and percussion drilling (Figs. 2 and 3; see also Fig. S1). 282 283 Fossil wave-built beach ridges were targeted by trenching in order to obtain a chronology 284 for the delta coast advance (Figs. 1 and 2; see also Fig. S1). Samples for Optically 285 Stimulated Luminescence (OSL) dating were collected where possible from within the 286 beach-foreshore facies in water tight opaque tubes (site I-11 at Labutta in the western side 287 of the delta; sites I-12 and I-13 at Seikma near the central delta coast; and site I-14 at 288 Kungyangon near the eastern delta coast). A sample was collected in the anthropogenic 289 overburden to date habitation on a Labutta beach ridge (Site I-10). In addition, two levee 290 samples were collected on meanders of the now defunct western major branch of the 291 Ayeyawady (Figs. 1 and 2; Table 2) near the apex of the delta (i.e., sites I-8 near Ta Loke 292 Htaw and I-9 near Lemyethna bordering the last abandoned course and an earlier-formed 293 oxbow lake, respectively). Levee samples were collected in trenches at the top of each 294 levee, below the overburden. 295 296 Drill coring was designed to recover continuous sediment records to the pre-deltaic 297 Pleistocene sediments (Figs. 2 and 3; see also Fig. S1; Table 1). Drill sites were located 298 in the middle and near the apex of the delta (site IR1 to 70.4 m depth at Kyonmangay 299 located 6.7 m above sea level and core IR2 to 43 m depth at Ta Loke Htaw located at 18 300 m above sea level, respectively) to assess the deltaic architecture and, in particular, how 301 far the post-glacial transgression reached inside the suspected Pleistocene Ayeyawady 302 incised valley. Facies analysis was based on the visual description of lithology, 303 sedimentary structures, textures and benthic foraminifera presence. In addition, XRF- 304 scanning-based high resolution chemostratigraphy was employed for the drill cores to 305 identify depositional environments using Woods Hole Oceanographic Institution’s 306 (WHOI) ITRAX XRF scanner (see methodology in Croudace et al., 2006). From the 307 suite of measured elements we used [Si]/[Rb] ratio to characterize the sand content (i.e., 308 Si-rich sand relative to fine grained muds, rich in Rb; Croudace and Rothwell, 2015), the 309 [Br]/[total XRF counts] ratio or Br* to characterize the organic matter (i.e., with Br 310 enriched in marine organic matter; McHugh et al., 2008), and [S]/[Rb] ratio to 311 characterize redox conditions in fine-grained muds (i.e., with S in excess of terrigenous 312 values in reducing conditions; Croudace and Rothwell, 2015). 313

8 314 Sediment sources for the pre-modern delta were estimated using radiogenic isotopes (Nd 315 and Sr) on a bulk sediment sample from the delta apex trench (I-8 taken as representative 316 for Ayeyawady fluvial sediment). To assess any potential addition of non-Ayeyawady 317 sediment sources (e.g., littoral drift, marine biogenic carbonates) another pre-modern 318 sample from the youngest dated fossil beach ridge trench near the coast (I-12) was 319 measured both as bulk and decarbonated sediment. The radiogenic composition of 320 sediments from the Sittaung River, the closest source to the delta other than Ayeyawady 321 iyself, was measured on a floodplain sample (Fig. ***). Nd and Sr chemistry was 322 undertaken with conventional ion chromatography following the method of Bayon et al. 323 (2002). Strontium was separated and purified from samples using Sr-Spec (Eichrom) 324 resin. Nd chemistry was performed with LN resin (Eichrom) following method described 325 in Scher and Delaney (2010). Sr and Nd analyses were conducted on the NEPTUNE 326 multi-collector ICP-MS at WHOI with the internal precision around 10–20 ppm (2σ); 327 external precision, after adjusting 87Sr/86Sr and 143Nd/144Nd values by 0.710240 and 328 0.511847 for the SRM987 and La Jolla Nd standards respectively, is estimated to be 15– 329 25 ppm (2σ).143Nd/144Nd isotopic composition is expressed further as εNd (DePaolo and 330 Wasserburg, 1976) units relative to (143Nd/144Nd)CHUR= 0.512638 (Hamilton et al., 331 1983). 332 333 Plant and wood pieces were radiocarbon-dated to derive a chronology for the deltaic 334 sediment succession and the pre-deltaic base (Table 1). Accelerator mass spectrometry 335 (AMS) radiocarbon dating was performed at the National Ocean Sciences Accelerator 336 Mass Spectrometry Facility (NOSAMS) at the WHOI. The methodology for AMS 337 radiocarbon dating is presented on the NOSAMS site (www.whoi.edu/nosams) and 338 discussed in McNichol et al. (1995). All dates have been converted to calendar ages using 339 CalPal 4.3 (Bronk Ramsey, 2009) and the IntCal13 calibration dataset (Reimer et al., 340 2013). 341 342 Seven samples were collected for OSL dating. Samples were collected using light-tight 343 metal tubes hammered horizontally into cleaned sediment surfaces. The tubes were 344 opened under subdued orange light at the Nordic Laboratory for Luminescence Dating 345 (Aarhus University) located at Risø (DTU Nutech) in Denmark. Using standard sample 346 preparation techniques (wet sieving, acid treatment, heavy liquids) purified quartz and K- 347 feldspar-rich extracts in the 180–250 µm grain size range were obtained (except sample 348 177202 for which it was 90-180 µm). Multi-grain aliquots of quartz and K-feldspar were 349 measured using a SAR protocol (Murray and Wintle, 2000) suitable for young samples. 350 The purity of the quartz OSL signal was confirmed using OSL IR depletion ratio test 351 (Duller, 2003; all aliquots within 10% of unity). For quartz OSL preheating for dose and 352 test dose was 200°C/10s and 160°C, respectively and K-feldspar rich extracts were 353 measured using a post-infrared (IR) Infrared Stimulated Luminescence (IRSL)

9 354 (pIRIR150) protocol based on Madsen et al. (2011). Early and late background 355 subtraction was used for quartz OSL and feldspar pIRIR dose calculations respectively. 356 Total dose rates to quartz and K-feldspar were calculated from radionuclide 357 concentrations measured on the outer material from the tubes using high resolution 358 gamma ray spectrometry (Murray et al., 1987). Samples were assumed to have been 359 saturated with water throughout the entire burial lifetime. 360 361 The morphology of the subaqueous extension of the Ayeyawady delta was studied using 362 the only available detailed bathymetric chart of the region that was based on surveys from 363 1850 to 1929 with small corrections until 1935 (British Admiralty, 1935). Newer 364 navigation charts of the region report only small corrections afterwards. The final DEM 365 (Figs. 4 and 5) consists of 6442 individual soundings reduced to the original datum at 366 Elephant Point at the entrance in Yangon River; to these we added the digitized 367 bathymetric contours of the original chart. To extend the bathymetry offshore beyond the 368 coverage of the original chart we used GEBCO 2014 Grid (General Bathymetric Charts 369 of the Oceans, a global 30 arc-second interval grid). Prior to digitizing, all charts and 370 satellite photos used in this study were georeferenced and transformed to a common 371 UTM projection (Zone 46 N) with Global Mapper 18.0 (http:// www.globalmapper.com/) 372 using 16 control points for each chart or photo. DEMs at a 250 m spatial resolution were 373 generated from digitized soundings with Surfer 12.0 software (Golden Software, Inc.). 374 The ‘‘natural neighbor’’ algorithm was chosen for interpolation because it is suitable for 375 a variable density of data across the interpolation domain and does not extrapolate depth 376 values beyond the range of existing data. 377 378 Results 379 380 In concert with satellite photos, our SRTM digital elevation model (Fig. 2a) reveals that 381 the morphologically-defined Ayeyawady delta plain starts immediately after the river 382 emerges from its mountainous valley at Myan Aung, bound on the western side by the 383 Indo-Burma Range and massive alluvial fans originating in the Bago Yoma on the 384 eastern side. Several inselberg-like pre-deltaic high terrains occur close to the coast on 385 the western side of Pathein River and on both sides of Yangon River. Two alluvial ridges, 386 5 to 7 m high relative to their adjacent delta plain, with visible meander belts and rare 387 crevasse splays, were constructed by large trunk channels (Fig. 2a, b, c). The western 388 alluvial ridge along the Daga course is largely fossil, whereas the eastern ridge is being 389 built along the present course of the river (Fig. 2c). Both ridges taper off in the mid-delta 390 as the trunk channels start to bifurcate into distributaries that split and rejoin on the lower 391 delta (Fig. 2a,b). After the bifurcation zone the delta plain is uniformly low in altitude (<5 392 m) with the exception of the higher mudflats near the entrance in the Sittaung estuary 393 (Fig. 2a). Although possessing meander belts of their own in their upper reaches, the 394 Pathein and Yangon Rivers, which are located at the western and eastern edge of the

10 395 delta, do not show visibly large alluvial ridges (Fig. 2a, b). This suggests that they were 396 not preferential routes for the main trunk Ayeyawady but secondary courses or have not 397 been active for very long. Near the coast, several generations of wave-built beach ridges 398 are evident in the lower part of the delta, bundling occasionally into beach ridge plains on 399 the Bogale Promontory and on the sides on Yangon River (Fig. 2b). 400 401 Sediment in our trenches on the Ayeyawady beach ridges exhibited weakly stratified, 402 mud-rich, fine sand lithologies. Fluvial deposits trenched near the apex showed a typical 403 levee facies exhibiting weakly laminated, amalgamated fine sands and muds below the 404 bioturbated and human-disturbed overburden. The IR1 drill core (Fig. 3) at Kyonmangay 405 (Fig. 2; see also Fig. S1) shows a succession of delta plain bioturbated soils and delta 406 plain muds overlaying amalgamated fine to medium sand and muds of the delta front and 407 prodelta/estuarine clayey muds with intercalated organic-rich detritus layers. Marine 408 influences are documented in the prodelta/estuarine and delta front deposits by high Br* 409 and rare benthic foraminifers. Tidal influence is indicated by thick-thin and sand-mud 410 alternations in the delta front deposits. Flooding is suggested by occasional clean sandy 411 layers in the prodelta facies. Both the delta plain and prodelta/estuarine deposits show 412 increased S/Rb values indicative of poorly oxic conditions. The transition to delta front 413 advance at Kyonmangay occurred at 13.5 m below sea level (mbsl) ~8,100 years ago, as 414 documented by the radiocarbon content of a leaf fragment. The deltaic succession stands 415 on a 9,300 years old mangrove peat at 28.5 mbsl near the base of the deltaic Holocene 416 deposits. Pre-Holocene fluvial deposits older than 10,200 years BP occur below, 417 consisting of structureless medium to coarse sands with clayey mud intercalations, 418 gravels, and fine-grained weakly laminated channel infills. 419 420 The IR2 drill core (Fig. 3) at Ta Loke Htaw (Fig. 2; see also Fig. S1) near the delta apex 421 on the modern alluvial ridge exhibits a succession of delta plain sandy muds topping 422 structureless medium sands with rare intercalated thin muds of channel/point bar type. 423 They overlie fine-grained, weakly laminated channel infill deposits and floodplain fine 424 sands with intercalated thin muds that started to accumulate ~8,900 years BP 425 (radiocarbon dated wood piece). Below ~25 mbsl structureless fine to medium sands of 426 channel/point bar and gravel layers occur to the base of the drill core. Organic material is 427 rare in all facies at Ta Loke Htaw except for occasional wood branches and a tree trunk in 428 the upper point bar facies. Marine influence is absent as foraminifers are not encountered 429 and Br* levels are consistently low. 430 431 The quartz OSL and feldspar pIRIR150 luminescence dating results are summarized in 432 Table 2 and Table S1. The quartz OSL signal is dominated by the fast component and the 433 average dose recovery ratio is 1.00±0.02 (4 samples, 11-12 aliquots per sample) 434 suggesting that our quartz De values measured using SAR are reliable. One prerequisite

11 435 for accurate age estimation is that the quartz OSL signal was sufficiently bleached prior 436 to burial in the sediment sequence. In this study we use the feldspar IR50 and pIRIR150 437 age data to provide insights into the completeness of bleaching of the quartz OSL signal 438 (e.g., Murray et al., 2012; Rémillard et al., 2016). This is based on the observation that 439 feldspar signals bleach much more slowly than quartz OSL (Godfrey-Smith et al., 1988; 440 Thomsen et al., 2008): IR50 signals bleach approximately one order of magnitude slower 441 than quartz OSL and pIRIR signals bleach even more slowly than IR50 signals (e.g., 442 Kars et al., 2014; Colarossi et al., 2015). We are confident that the quartz signal is well- 443 bleached when the pIRIR150 age agrees within uncertainty with the quartz age; this is the 444 case for sample 177204. We consider that the quartz OSL signal is very likely to be 445 completely bleached when the IR50 age agrees or is slightly lower (due to fading) than 446 the quartz age. This is the case for all samples except for sample 177202 for which the 447 IR50 age may be slightly older. Nevertheless, this does not mean the quartz OSL age for 448 this particular sample is affected by partial bleaching; we just cannot be certain it is not. 449 450 Overall, optical ages on the natural levee of an old meander series of the fossil eastern 451 alluvial ridge indicate full activity by ~1,750±320 years ago. Sedimentation on the top of 452 the natural levee bordering the last Daga course indicates that its abandonment took place 453 no earlier than 1,500±230 years ago (Fig. 2; see also Fig. S1). A radiocarbon date 454 calibrated to ~1,300 years ago on a large wood trunk from the point bar facies drilled at 455 Ta Loke Htaw indicates that the present eastern course of the Ayeyawady was active at 456 the time. The fresh appearance of the wood make it unlikely that it is remobilized fossil 457 wood. Future systematic exploration of the meander belts subsurface architecture is 458 needed to reconstruct their history. 459 460 Our combined chronology indicates the Ayeyawady delta reached as far south as the 461 latitude of the cities of Yangon and Pathein around 6,300 years ago, as documented by a 462 radiocarbon content of a leaf fragment from the delta plain facies at Kyonmangay. 463 Optical dating shows that the least advanced beach ridge bundle found on the western 464 side of the delta near Labutta is also the oldest (~4,600 years old; Fig. 2 and Fig. S1). The 465 beach ridge plain at the Bogale Promontory started ~1,000 years ago, soon after beach 466 ridges started to form at the Yangon River mouth (~1,200 years ago; Fig. 2 and Fig. S1). 467 468 Radiogenic provenance fingerprinting of the bulk river sediment (Table S2) on the Ta 469 Loke Htaw levee shows that 143Nd/144Nd (εNd) and 87Sr/86Sr values of 0.512263 (-7.3) 470 and 0.7120 respectively, lie close to the beach ridge sediment composition: 0.512285 (- 471 6.9) and 0.7118 for bulk sediment and 0.512287 (-6.8) and 0.7119 for bulk decarbonated 472 sediment. The identical 87Sr/86Sr values for the bulk and decarbonated beach ridge sample 473 suggest that marine biogenic carbonates are a minor sediment component at the coast. 474 However, previous measurements on Ayeyawady sediments (Table S2 with data from

12 475 Allen et al., 2008 – 150 km upstream of the delta; Colin et al., 1999 – at an unspecified 476 location) show a larger variability in εNd with values of -8.3 and -10.7. The closest 477 sediment source along the coast, the Sittaung River that drains Bago Yoma and the Shan 478 Plateau shows 143Nd/144Nd (εNd) and 87Sr/86Sr values of 0.512105 (-10.4) and 0.7168 479 respectively. The Yangon River, the largely abandoned easternmost branch of the 480 Ayeyawady close to Bago Yoma has εNd and 87Sr/86Sr of -12.2 and 0.7080 respectively 481 (Damodararao et al., 2016), which suggest mixing with a source similar to the Sittaung. 482 483 Our reassessment of the late 19th – early 20th century bathymetry with the high-resolution 484 digital elevation model produced several surprises (Figs. 3 and 4a). First, the edge of the 485 shelf (Fig. 4) was found to be significantly deeper in front of the Mottama Depression 486 (>150 m) than west of it (100–120 m deep). Second, the mud belt along the Ayeyawady 487 delta exhibits a clinoform attached to the shore and likely composed of sandy muds (Rao 488 et al., 2005) and extending to depths of 35–40 m. In contrast, the Gulf of Mottama 489 exhibits a thick mid-shelf clinoform comprised of finer muds (Rao et al., 2005) with the 490 steep frontal region extending from 40 to 90 m water depth. The transition between the 491 western and eastern clinoforms is marked by a transversal channel that is 10 km wide and 492 5 m deep on average and is flanked on the deeper eastern side by a drift-like elongated 493 feature of similar average dimensions. Third, a flatter area of the outer shelf in front of 494 the western Ayeyawady delta coast stands out from the typical outer shelf chaotic relief, 495 suggesting potential preservation of a relict pre-Holocene delta region at water depths 496 between 35 and 45 m. 497 498 Discussion 499 500 Our new drill core information (Fig. 3) indicates that the Holocene Ayeyawady delta 501 advanced into an incised valley estuarine embayment that extended north of 502 Kyonmangay (~80 km from the current coast) but did not reach as far as the current delta 503 apex at Ta Loke Htaw (270 km from the coast). The Pleistocene deposits of the incised 504 valley intercepted in our cores are fluvial, generally much coarser than the delta deposits 505 but heterolithic with indications of increasing tidal influence nearer to the Andaman Sea 506 at Kyonmangay. The overlying peats atop mudflat sediments sampled at Kyonmangay 507 indicate the presence of a muddy coast with mangroves at the time of their transgression 508 ~9,300 years ago. Given that the contemporaneous ice-volume equivalent global sea level 509 was between -29 and -31 mbsl (Lambeck et al., 2014), the altitude of the mangrove peat 510 (-28.3 mbsl) on the largely incompressible Pleistocene deposits below indicate that the 511 delta is vertically stable. However, glacial isostatic adjustment modeling is needed to 512 quantify subsidence as neighboring regions of Thailand and Malay Peninsula (Bradley et 513 al., 2016) suggest that relative sea level reached higher earlier during the deglaciation. 514 After the mangrove coast was flooded, the marine embayment accumulated 515 estuarine/prodelta muds afterwards. At 8,100 years ago the Ayeyawady bayhead delta

13 516 front reached the southern Kyonmangay site and by ~6,300 years ago delta plain 517 deposition started. 518 519 Deposits at the delta apex in the drill core at Ta Loke Htaw indicate a dynamic fluvial 520 environment with channel erosion (i.e., scouring) followed by point bar and floodplain 521 deposition. The abandonment of the western Daga meander belt after 1,500 years ago, 522 left the Ayeyawady flowing along a single preferential course. Meander belt construction 523 on the old and new course of the river, leading to the formation of alluvial ridges, appears 524 to be an efficient type of aggradation on the upper delta plain before the river starts to 525 bifurcate. 526 527 Near the coast, the quasi-contemporary beach ridge development on the Bogale 528 Promontory and Yangon River mouth argue for the advanced position of the western half 529 of the delta being acquired early and maintained during progradation. Delta growth since 530 6,300 years ago, with intermediate stages delineated by successive beach ridge sets, point 531 to decreasing rates of advance of ~25 m/year until ~4,600 years ago and 8 to 10 m/year 532 afterwards. The latter are still higher than the average progradation value of 3.4 m/year 533 calculated by Hedley et al. (2010) for the last century or so. Furthermore, the recent 534 progradation occurred primarily on the coast adjacent to both sides of the Yangon River, 535 while the shoreline of the rest of the delta has been largely immobile. It is important to 536 note that, like the Ayeyawady, many large river deltas developing under the Asian 537 monsoon regime, such as Mekong (Ta et al., 2002), Red River (Tanabe et al., 2003), or 538 Godavari (Cui et al., 2017) started to form wave-built beach ridges between 5000 and 539 4000 years ago changing from river-dominated morphologies to show stronger wave- 540 influenced characteristics. Given that these deltas were at various stages of advance from 541 within their incised valleys onto the shelf it is more likely that their morphological 542 evolution was climatically driven rather than controlled by local factors as previously 543 proposed. As the late Holocene monsoon aridification started at that time (Ponton et al., 544 2012; Dixit et al., 2014), fluvial discharge variability at centennial timescales increased 545 setting the stage for periodic wave-dominance of deltaic coasts during more arid 546 intervals. 547 548 Our re-evaluation of the shelf morphology in the context of the new data onland reveals 549 important information for understanding the peculiar, irregular growth of the Ayeyawady 550 delta. Its western half from Cape Maw Deng to the Bogale Promontory is well advanced 551 into the Andaman Sea in comparison to its eastern half. First, the shelf DEM suggests 552 that the western Ayeyawady delta continues offshore into a shallow, coarser-grained 553 shore-attached clinoform, which is not completely unexpected given the relatively low 554 tidal range of 2–3 m (e.g., Goodbred and Saito, 2012) and the perennial loss of sediment 555 advected to the Gulf of Mottama (Ramaswamy and Rao, 2014). The Nd and Sr

14 556 fingerprint of the river sediment is almost identical to the beach ridge at Bogale 557 indicating that essentially no sediment from the Gulf of Mottama bearing the radiogenic 558 imprint of Sittaung (see above) and especially Thanlwin (Damodararao et al., 2016) is 559 feeding this part of the coast. The shore-attached sandy clinoform tapers off after 40 mbsl 560 (Fig. 2b). In contrast, the Gulf of Mottama exhibits a mid-shelf mud clinoform with the 561 roll-over at 40 m and toe depth of 80–90 m. The internal architecture of this distinctive 562 feature was imaged previously (Ramaswamy and Rao, 2014) and showed seismic 563 characteristics typical of a clinoform topset and foreset. High rates of 564 progradation/aggradation for the Mottama clinoform have been suggested previously but 565 a core collected on its lower foreset has an average sedimentation rate of ~1 cm/year 566 since ~1450 AD (Ota et al., 2017), which is one order of magnitude less than proposed 567 before (Chhibber, 1934; Rodolfo, 1975). Given the depressed character of the Mottama 568 shelf, as indicated by the shelf edge position 40 to 70 m lower than in front of the western 569 Ayeyawady delta, perhaps, it is not surprising that infilling of this region is still ongoing. 570 What is surprising instead is why and how the Ayeyawady River built its delta on the 571 eastern raised shelf block rather than in advancing preferentially into the Gulf of 572 Mottama. Such behavior defies theoretical and modeling expectations of a more 573 advanced deltaic coast toward the subsided block (e.g., Liang et al., 2016). The key to 574 this problem appears to be again suggested by the shelf morphology. 575 576 The distinctive transition between clinoforms exhibiting a wide elongated channel and 577 what appears to be an attached sediment drift-like feature suggests intense current activity 578 at the common clinoform boundary. Indeed tidal modeling suggests that a tidal shear 579 front (e.g., Wang et al., 2017) may be present in this region that shows a drastic change 580 from weak and isotropic tidal currents west of Bogale Promontory to highly oriented 581 strong currents in the Gulf of Mottama (Rizal et al., 2012). Such a shear front would 582 explain both the unusual channel-drift couplet, but also the fact the Ayeyawady was able 583 to build its delta west of the gulf. If the tidal shear front has been a long-lived feature of 584 the shelf circulation then it probably acted as a littoral energy fence (sensu Swift and 585 Thorne, 1992) trapping a significant part of the Ayeyawady sediment on the raised 586 western shelf block. However, such an energy fence may be broken by prevailing 587 westerly currents during the summer monsoon when water and sediment discharge peaks 588 from the Ayeyarwady to provide finer suspended sediment to the Mottama clinoform. 589 Given the depressed character of the Mottama shelf block, the front must have existed 590 since the beginning of the deglacial transgression of the northern Andaman shelf. 591 Industrial seismic reflection profiles imaged a region of strike-slip extension in the Gulf 592 of Mottama expressed as horsetail extensional splays linked to the Sagaing Fault system 593 (Morley, 2017) that can explain the height differential between the western and eastern 594 shelves. Furthermore, the shear front must have gradually intensified through positive 595 feedback with the morphology as the shore-attached clinoform west of it grew larger. In

15 596 contrast, the amplified tidal currents in the Gulf of Mottama efficiently redistributed the 597 significantly larger amount of Ayeyawady sediments that escaped beyond the energy 598 fence together with sediments from the Sittaung and Thanlwin to form the midshelf 599 clinoform there. The offshore-directed tidal pumping leading to the formation of the 600 Mottama clinoform is reminiscent of the situation on the eastern Indus shelf where strong 601 tidal currents from the Gulf of Kutch built a mid-shelf clinoform with Indus sediments 602 escaping eastward (Giosan et al., 2006). Such clinoforms, which are of purely tidal 603 origin, and do not front a subaerial deltaic counterpart per se may have been more 604 common in sediment-rich macrotidal environments during faster transgressive conditions 605 in the past. 606 607 Conclusions 608 609 The Ayeyawady delta in Myanmar is the last realization in a long series of depocenters 610 that gradually moved southward within the tectonically dynamic intra-mountainous 611 landscape extending from the Central Myanmar Basin in the north to the northern 612 Andaman Sea in the south (Figs. 1 and 2). The delta developed within the incised valley 613 dug by the Ayeyawady River during the last lowstand (Fig. 3). The Pleistocene valley 614 was flooded at least 80 km inland from the present coast during the deglacial sea level 615 rise. Holocene progradation into this paleo-Ayeyawady bay proceeded in the form of a 616 fluvial- and tide-dominated delta until late Holocene wave action began to build isolated 617 and clustered beach ridges at the contemporaneous coasts (Fig. 2). However, beach ridges 618 are rather rare and underdeveloped, testifying to the enormous sediment load discharged 619 by the Ayeyawady and tidal dispersal and reworking. Ridge construction during the late 620 Holocene, similar to several other deltas across the Indian monsoon domain, argues for a 621 possible climatic control on delta morphodynamics through variability in discharge, 622 changes in wave climate, or both. 623 624 The landscape near the delta apex exhibits active and fossil late Holocene meander belts 625 that terminate in the mid-delta where the discharge is split to lower order distributary 626 channels (Fig. 2). The meander belts stand as alluvial ridges above the floodplain along 627 the active river course, as well as its antecedent paleo-course documenting the 628 Ayeyawady’s avulsive character. Construction of a more advanced coast in the western 629 half of the delta could be seen as a quasi-independent region, the Pathein lobe (Fig. 5), 630 which was probably favored by the more western location of the early course of the river 631 (but see below). The eastern region of the delta (the Yangon lobe) is offset inland (Fig. 5) 632 and exhibits a more wave-dominated morphology, largely built with Ayeyawady-derived 633 sediment escaping alongshore. Further east, the Yangon lobe merges with the mudflats 634 fringing the Sittaung estuary (Fig. 5). Despite its large sediment load the Thanlwin River 635 has only built a bayhead delta, barely prograding outside its incised valley, probably due

16 636 to extreme macrotidal conditions at its mouth (Fig. 5). However, its sediment instead 637 contributed to deposition on the shelf, as did part of the load from both Ayeyawady and 638 Sittaung. 639 640 Correlation of the delta morphological and stratigraphic architecture information onland 641 to the shelf bathymetry and hydrodynamics, as well as its tectonic and sedimentary 642 characteristics, provides insight on the peculiar growth style of the Ayeyawady delta 643 (Figs. 2–5). The offset between the western Pathein lobe and the eastern deltaic coast 644 appears to be driven by tectonic-hydrodynamic feedbacks as the extensionally lowered 645 shelf block of the Gulf of Mottama amplifies tidal currents relative to the eastern part of 646 the shelf. This situation probably activates a perennial shear front between the two 647 regions that behaves as a leaky energy fence. Just as importantly, the strong currents in 648 the Gulf of Mottama act as an offshore-directed tidal pump that help build a deep, mixed- 649 source mid-shelf clinoform, the Ayeyawady-Sittaung-Thanlwin subaqueous delta, into 650 the Mottama shelf depression. 651 652 Our study takes a first look at the evolution of the Holocene Ayeyawady delta to provide 653 a basis for more detailed work and context to present and future management plans for 654 this ecologically and economically important, but vulnerable region. A first conclusion 655 for the future of the region comes by comparing the Ayeyawady to other deltas across the 656 world. Uniquely for deltas of its size the Ayeyawady delta has not suffered a sediment 657 deficit from damming, yet it has been barely growing. The reason is the highly energetic 658 tidal, wind and wave regime of the northern Andaman Sea that exports most sediments 659 offshore despite the large load of the river as envisioned by Ramswamy et al., (2004) and 660 Hedley et al. (2010). In addition to their effects upstream (Brakenridge et al., 2017), the 661 expected sediment deficit after dams are constructed on the river and tributaries may 662 significantly impact the delta fragile sedimentary equilibrium (Giosan et al., 2014). This 663 could make it more vulnerable to the accelerating sea level rise (Syvitski et al., 2009) or 664 changes in frequency and intensity of cyclones hitting the coast (Darby et al., 2016) that 665 compound with increased subsidence linked to the rapid development of the region (e.g., 666 Van der Horst, 2017). 667 668

17 669 Acknowledgments 670 671 We are greatful to reviewers Kelvin Rodolfo and Torbjörn Törnqvist for their 672 suggestions. This study was primarily supported by an Andrew W. Mellon Foundation 673 Award for Innovative Research from the Woods Hole Oceanographic Institution to L. 674 Giosan. Additional funds were provided by the Charles T. McCord Chair in Petroleum 675 Geology to P. Clift. We thank Myanmar authorities for project permissions as well as 676 leaders and residents of villages that we visited in the Ayeyawady delta for hospitality 677 and help. We also thank V. Ramaswamy (NIO, Goa) for providing inspiration with his 678 previous Andaman Sea work and for help with initial contacts in Myanmar. N. Khonde 679 gratefully acknowledges the SERB Indo-US Postdoctoral Fellowship sponsored by 680 SERB-IUSSTF for research work at Woods Hole Oceanographic Institution, USA. 681

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21 817 Racey, A. and Ridd, M.F.: Petroleum Geology of Myanmar. Geological Society of 818 London, 2015. 819 Ramaswamy V., Rao P. S., Rao K. H., Thwin, S., Rao, Srinivasa N. and Raiker, V.: Tidal 820 influence on suspended sediment distribution and dispersal in the northern Andaman 821 Sea and Gulf of Martaban Marine Geology 208 33–42, 2004. 822 Ramaswamy, V., and Rao, P. S.The Myanmar continental shelf. In F. L. Chiocci & A. R. 823 Chivas (Eds.), Continental Shelves of the World: Their Evolution During the Last 824 Glacio-Eustatic Cycle (pp. 231-240). Bath, UK: Geological Society of London, 2014. 825 Rao, P. S., Ramaswamy, V. and Thwin, S.: Sediment texture, distribution and transport 826 on the Ayeyarwady continental shelf, Andaman Sea. Mar. Geol. 216, 239–247 2005. 827 Reimer P. J., Bard, E., Bayliss, A., Beck, J. W., Blackwell, P. G., Bronk, Ramsey C, 828 Buck, C. E., Edwards, R. L., Friedrich, M., Grootes, P. M., Guilderson, T. P., 829 Haflidason, H., Hajdas, I., Hatté, C., Heaton, T. J., Hoffman, D. L., Hogg, A. G., 830 Hughen, K. A., Kaiser, K. F., Kromer, B., Manning, S. W., Niu, M., Reimer, R. W., 831 Richards, D. A., Scott, M., Southon, J. R., Staff, R. A., Turney, C. S. M., van der 832 Plicht, J.: IntCal13 and Marine13 radiocarbon age calibration curves 0–50,000 years 833 cal BP. Radiocarbon 55(2): 1869-1887, 2013. 834 Rémillard, A.M., St-Onge, G., Bernatchez, P., Hétu, B., Buylaert, J.-P., Murray, A.S., 835 Vigneault, B., 2016. Chronology and stratigraphy of the Magdalen Islands 836 archipelago from the last glaciation to the early Holocene: new insights into the 837 glacial and sea-level history of eastern Canada. Boreas, 45, 604-628. 838 Rizal, S., Damm, P., Wahid, M.A., Sundermann, J., Ilhamsyah, Y. and Iskandar T.: 839 General circulation in the Malacca Strait and Andaman Sea: A numerical model 840 study. American Journal of Environmental Sciences. 8. 479-488. 841 10.3844/ajessp.2012.479.488. , 2012. 842 Robinson, R. A. J., Bird, M. I., Oo, N. W., Hoey, T. B., Aye, M.M., Higgitt, D. L., Swe, 843 A., Tun, T. and Win, S.L.: The Irrawaddy river sediment flux to the Indian Ocean: the 844 original nineteenth-century data revisited. The Journal of Geology, 115(6), 629-640, 845 2007. 846 Rodolfo, K. S., 1969a, Bathymetry and marine geology of the Andaman Basin, and 847 tectonic implications for Southeast Asia Geological Society of America Bulletin 80: 848 1203–1230 849 Rodolfo, K. S., 1969b, Sediments of the Andaman Basin, Northeastern Indian Ocean. 850 Mar. Geol. 7, 371– 402. 851 Rodolfo, K.S., 1975. The Irrawaddy Delta: Tertiary setting and modern offshore 852 sedimentation. In Deltas: Models for Exploration, Broussard ML (ed.). Houston 853 Geological Society: Houston; 329–348. 854 Scher H. D. and M. L.: Delaney, Braking the glass ceiling for high resolution Nd records 855 in early Cenozoic paleoceanography, Chemical Geology 269, 269-329, 2010.. 856 Seekins, D. M.: “State, society and natural disaster: cyclone Nargis in Myanmar 857 (Burma)”, Asian Journal of Social Science, 37(05), pp.717-37, 2009. 858 Shi, W. and Wang, M.: Three-dimensional observations from MODIS and CALIPSO for 859 ocean responses to cyclone Nargis in the Gulf of Martaban. Geophysical Research 860 Letters, 35, L21603, 2008. 861 Stamp, D. L. The Irrawaddy River. Geogr. J. 5, 329–352, (1940).

22 862 Swift, D. J. P. and Thorne, J. A.: Sedimentation on Continental Margins, I: A General 863 Model for Shelf Sedimentation, in Shelf Sand and Sandstone Bodies: Geometry, 864 Facies and Sequence Stratigraphy (eds D. J. P. Swift, G. F. Oertel, R. W. Tillman and 865 J. A. Thorne), Blackwell Publishing Ltd., Oxford, UK. doi: 866 10.1002/9781444303933.ch1, 1992. 867 Syvitski, J. P., Kettner, A. J., Overeem, I., Hutton, E.W., Hannon, M.T., Brakenridge, 868 G.R., Day, J., Vörösmarty, C., Saito, Y., Giosan, L. and Nicholls, R.J., Sinking deltas 869 due to human activities. Nature Geoscience, 2(10), 681-686, 2009. 870 Ta, T. K. O., Nguyen, V. L., Tateishi, M., Kobayashi, I., Saito, Y. and Nakamura, T., 871 Sediment facies and Late Holocene progradation of the Mekong River Delta in Bentre 872 Province, southern Vietnam: an example of evolution from a tide-dominated to a tide- 873 and wave-dominated delta. Sedimentary Geology, 152(3), 313-325, 2002. 874 Taft, L. and Evers, M.: A review of current and possible future human-water dynamics in 875 Myanmar's river basins. Hydrological Earth System Science 20: 4913-4928, 2016. 876 Tanabe, S., Hori, K., Saito, Y., Haruyama, S. and Kitamura, A.: Song Hong (Red River) 877 delta evolution related to millennium-scale Holocene sea-level changes. Quaternary 878 Science Reviews, 22(21), 2345-2361, 2003. 879 Thomsen, K., Murray, A. S., Jain, M. & Bøtter-Jensen, L. 2008: Laboratory fading rates 880 of various luminescence signals from feldspar-rich sediment extracts. Radiation 881 Measurements 43, 1474–1486. 882 Van der Horst, T., 2017. Sinking Yangon: Detection of subsidence caused by 883 groundwater extraction using SAR interferometry and PSI time-series analysis for 884 Sentinel-1 data. MS Thesis, Delft University of Technology and the National 885 University of Singapore, http://repository.tudelft.nl 886 Volker, A.: The deltaic area of the Irrawaddy river in Burma in Scientific problems of the 887 humid tropical zone deltas and their implications Proceedings of the Dacca 888 Symposium UNESCO 373–9, 1966. 889 Wang, N., Li, G., Qiao, L., Shi, J., Dong, P., Xu, J. and Ma, Y.: Long-term evolution in 890 the location, propagation, and magnitude of the tidal shear front off the Yellow River 891 Mouth. Continental Shelf Research, 137, 1-12, 2017. 892 893

23 894 Table 1. Results of AMS 14C dating of organic materials from drill cores IR1 (Kyonmangay) and IR2 (Ta Loke Htaw). 895 896 Location Sample Altitude Type Labcode Latitude Longitude Age Error d13C Calibrated Age Error (m bsl) (years BP) (years BP) (per mil) (years)* (years) Observations Kyonmangay IR1-9.60 -2.9 leaf fragment OS-132754 16°26'15N 95°08'01"E 5,590 100 -28.65 6487 213 small Kyonmangay IR1-20.0 -13.3 leaf fragment OS-132658 16°26'15N 95°08'01"E 7,300 40 -26.71 8166 80 Kyonmangay IR1-35.0 -28.3 mangrove wood piece OS-133490 16°26'15N 95°08'01"E 8,300 40 -27.27 9352 148 Kyonmangay IR1-40.0 -33.3 carbonized wood piece OS-132659 16°26'15N 95°08'01"E 9,100 35 -26.58 10351 88.5

Ta Loke Htaw IR2-19.0 -0.5 wood trunk piece OS-133606 17°39'13"N 95°26'2"E 1,320 15 -28.04 1307 53.5 Ta Loke Htaw IR2-33.5 -15.0 carbonized wood piece OS-135132 17°39'13"N 95°26'2"E 8,020 30 -27.7 8959 117.5 small 897 *Calendar ages are relative to year 2016 898

24 899 Table 2. Summary of the quartz and feldspar luminescence data. (n) denotes the number of aliquots contributing to dose (De). The 900 saturated water content (w.c.) is given as the ratio of weight of water to dry sediment weight. Feldspar IR50 and pIRIR150 ages have 901 not been corrected for any signal instability. Radionuclide concentrations used to derive quartz and feldspar dose rates are given in 902 Table S1. Bleaching of quartz OSL signal is assessed by comparing the quartz ages with the IR50 and pIRIR150 ages. Uncertainties 903 represent one standard error. Age uncertainties include random and systematic components. Quartz ages should be used for 904 interpretation; feldspar ages are only used to investigate quartz OSL bleaching. 905 906

Quartz Quartz pIRIR150 IR 50 Quartz pIRIR150 IR 50 Quartz K-feldspar Sample Latitude / Depth, well- Dose rate, Dose rate, w.c. code Site Setting Longitude cm bleached? Age , ka Age , ka Age , ka Dose , Gy (n) Dose , Gy Dose , Gy (n) Gy/ka Gy/ka %

fluvial N 17 38 36.82 / 17 72 01 I8 levee E 95 18 33.64 95 probably 1.50 ± 0.23 6.64 ± 0.68 1.20 ± 0.23 3.28 ± 0.49 34 20.6 ± 1.9 3.73 ± 0.69 9 2.19 ± 0.10 3.10 ± 0.12 29 fluvial N 17 36 13.5 / 17 72 02 I9 levee E 95 12 53.39 110 not certain 1.75 ± 0.32 15.2 ± 5.5 4.02 ± 1.85 4.14 ± 0.73 35 46 ± 16 12.0 ± 5.5 9 2.37 ± 0.10 2.99 ± 0.11 35

beach N 16 09 03.5 / 17 72 03 I10 ridge E 94 43 57.3 92 probably 1.46 ± 0.22 2.35 ± 0.21 1.10 ± 0.07 2.97 ± 0.42 40 6.9 ± 0.5 3.25 ± 0.17 9 2.03 ± 0.09 2.95 ± 0.11 28 beach N 16 09.2578 / 17 72 04 I11 ridge E 94 44.1843 90 confident 4.63 ± 0.47 4.73 ± 0.37 2.71 ± 0.17 10.1 ± 0.9 36 14.7 ± 1.0 8.42 ± 0.43 9 2.18 ± 0.09 3.10 ± 0.11 32

beach N 15 50 10.5 / 17 72 05 I12 ridge E 95 29 51 100 probably 1.04 ± 0.09 1.94 ± 0.19 0.79 ± 0.05 2.64 ± 0.17 38 6.7 ± 0.6 2.72 ± 0.14 9 2.53 ± 0.13 3.45 ± 0.15 38 beach N 15 49.6494 / 17 72 06 I13 ridge E 95 30.2095 132 probably 0.86 ± 0.07 1.86 ± 0.15 0.68 ± 0.04 1.58 ± 0.12 37 5.1 ± 0.4 1.88 ± 0.08 9 1.84 ± 0.07 2.75 ± 0.10 40

beach N 16 24 27.5 / 907 17 72 07 I14 ridge E 96 02 20.2 115 probably 1.19 ± 0.11 1.43 ± 0.12 0.76 ± 0.04 2.64 ± 0.19 40 4.5 ± 0.3 2.38 ± 0.09 9 2.21 ± 0.10 3.13 ± 0.12 24

908 Fig. 1. Physiography (left) and geology (right) of the Ayeyawady Basin and adjacent regions. 909

910 911 912 913 914 915

916 Fig. 2. (a) SRTM-derived DEM for the Ayeyawady delta region (pattern of colors repeats 917 every 10 m to 300 m in height; higher landscape in black); (b) large scale features of the 918 Ayeyawady delta region with identified river and distributary courses and mouths as well as 919 beach ridges shown on an ASTER satellite photo ; (c) sample locations and chronology on the 920 meander belts documenting the avulsion near the delta apex (meander belts as white lines 921 delimited from ASTER and Google Earth images); (d) Preliminary model of the Ayeyawady 922 delta evolution with sampling locations and types with chronological information on the 923 youngest fluvial deposits and beach ridges. 924

925 926

927 Fig. 3. (a) Depositional environments interpreted from litho- and chemo-stratigraphy with 928 radiocarbon chronology for drill cores in the Ayeyawady delta; (b) Interpreted Ayeyawady 929 delta stratigraphy and evolution along the Ayeyawady’s main course. 930

931 932 28

933 Fig. 4. Interpreted bathymetric profiles across the northern Andaman Sea shelf (bathymetric 934 profiles identified on map). Dashed line on map indicates the approximate limit of consistent 935 fine-grained sediment deposition on the shelf farthest from shore. The SRTM-derived DEM 936 for the Ayeyawady delta region is shown onland. 937

938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 29

955 Fig. 5. (Upper) Bathymetry of the northern Andaman Sea shelf and SRTM-derived DEM for 956 the Ayeyawady delta region onland with regional faults and associated splay faults (Morley, 957 2017); arrow pairs indicate regional compression (white) or extension (red); (Middle) Tidal 958 range lines (black), co-tidal lines (white) and tidal current magnitudes (ellipses) for the 959 dominant M2 tide component (Rizal et al., 2012); (Lower) Sketch he Ayeyawady delta plain 960 evolution phases and associated subaqueous deltas. 961

962 30